Closed loop hydrostatic circuits are conventionally used for actuating components in mobile heavy equipment. In a typical closed loop hydrostatic system, a driving pump is connected to provide pressurized fluid communication to an actuator, such as a hydraulic cylinder or hydraulic motor. To affect reversible actuation, the system is switchable so that the driving pump can selectively provide pressure in opposite directions to the actuator.
More specifically, in a conventional system wherein the actuator is a hydraulic motor, the fluid flow through the motor is directed to return to the driving pump. When the flow direction from the driving pump is reversed, the rotation direction of the motor is reversed. The speed of rotation of the motor is dependent on the flow rate provided by the driving pump.
Also, in a conventional system wherein the actuator is a hydraulic cylinder, the driving pump provides pressurized flow to a base side of the cylinder (against a top side of the piston) to cause a reciprocating rod to extend. The rod is caused to retract by providing pressurized flow into a rod side of the cylinder.
Some amount of internal fluid leakage occurs in a real hydraulic systems. In the hydraulic motor system, for example, leakage occurs at the high pressure side of the motor. This leaked fluid is typically returned to the system reservoir through a case drain port in the motor. However, because of this leakage, fluid returns to the pump at a slightly lower flow rate than the fluid supplied to the motor from the pump.
Additional factors in a hydraulic cylinder system result in an even greater flow rate differential. In particular, the cylinder has a substantially higher maximum volume at a piston side than at a rod side, due to a volume occupied by the rod itself. Therefore, the flow rate returning to the pump from the piston side when the rod is retracting is substantially greater than the flow rate entering the rod side of the cylinder. Because of this flow rate differential, excess flow is conventionally returned to the reservoir through a relief valve during rod retraction. Conversely, when the rod is being extended, a flow rate returning to the pump from the rod end of the cylinder is substantially less than the flow rate entering the base end of the cylinder from the pump.
In these instances where an input/output flow rate differential exists, the flow rate deficiency is compensated by a make-up and/or control fluid flow known as "charge" flow. In known systems, the charge flow is supplied by a charge pump integrally mounted in the main pump.
Typically, in the conventional main pump, a driving pump and integral charge pump are driven by a common shaft. However, because the conventional charge pump is mechanically driven by the same shaft as the driving pump, the integral charge pump has a fixed rate output corresponding to a given rotational speed of the driving pump. Problems occur in conventional charging systems because the charge pump flow rate is directly dependent on a corresponding rotational speed of the driving pump.
A hydraulic system with a conventional charge pump can fail to meet an upper range of desired actuation speeds. The charge flow rate demand increases with actuation speed, particularly in the hydraulic cylinder system wherein the bottom piston area at the rod side is smaller than a top piston area at the opposite side. Unfortunately, when higher actuation speeds are approached, the charge flow rate demand exceeds the fixed flow rate output capacity of a conventional integral charge pump, resulting in cavitation within the charge pump and an undesirably low peak in actuation speed performance.
The charge flow rate must be increased in order to increase actuation speed. However, to redesign a conventional integral charge pump to provide a high flow rate capacity would be impractical because the nature of integral charge pumps necessitates the sacrifice of high actuation speeds in the interest of economy. Specifically, the need for charge flow fluctuates depending on the crane operation being performed, but the conventional charge pump generates charge flow output whether or not the system needs it, and excess flow is diverted to the fluid reservoir by a relief valve. This generation of excess charge flow wastes energy for running the charge pump, steals power away from the main pump, and unnecessarily heats the hydraulic fluid. Because an integral charge pump designed to support high actuation speeds would exhibit exaggerated inefficiencies, practical design considerations dictate a compromise between efficiency and actuation speed. As a result, a charge pump is typically engineered to have an output capacity in an intermediate range, resulting in slower-than-desired peak actuation speeds.
Therefore, a need exists for a means to efficiently provide a high charge flow rate in order to achieve high actuation speeds.